Human food and animal feed derived from many grains are deficient in the sulfur amino acids, methionine and cysteine, which are required in an animal diet. In corn, the sulfur amino acids are the third most limiting amino acids, after lysine and tryptophan, for the dietary requirements of many animals. The use of soybean meal, which is rich in lysine and tryptophan, to supplement corn in anmial feed is limited by the low sulfur amino acid content of the legume. Thus, an increase in the sulfur amino acid content of either corn or soybean would improve the nutritional quality of the mixtures and reduce the need for further supplementation through addition of more expensive methionine.
Efforts to improve the sulfur amino acid content of crops through plant breeding have met with limited success on the laboratory scale and no success on the commercial scale. A mutant corn line which had an elevated whole-kernel methionine concentration was isolated from corn cells grown in culture by selecting for growth in the presence of inhibitory concentrations of lysine plus threonine Phillips et al. (1985) Cereal Chem. 62:213-218!. However, agronomically-acceptable cultivars have not yet been derived from this line and commercialized. Soybean cell lines with increased intracellular concentrations of methionine were isolated by selection for growth in the presence of ethionine Madison and Thompson (1988) Plant Cell Reports 7:472-476!, but plants were not regenerated from these lines.
The amino acid content of seeds is determined primarily by the storage proteins which are synthesized during seed development and which serve as a major nutrient reserve following germination. The quantity of protein in seeds varies from about 10% of the dry weight in cereals to 20-40% of the dry weight of legumes. In many seeds the storage proteins account for 50% or more of the total protein. Because of their abundance, plant seed storage proteins were among the first proteins to be isolated. Only recently, however, have the amino acid sequences of some of these proteins been determined with the use of molecular genetic techniques. These techniques have also provided information about the genetic signals that control the seed-specific expression and the intracellular targeting of these proteins.
One genetic engineering approach to increase the sulfur amino acid content of seeds is to isolate genes coding for proteins that are rich in the sulfur-containing amino acids methionine and cysteine, to link the genes to strong seed-specific regulatory sequences, to transform the chimeric gene into crops plants and to identify transformants wherein the gene is sufficiently-highly expressed to cause an increase in total sulfur amino acid content. However, increasing the sulfur amino acid content of seeds by expression of sulfur-rich proteins may be limited by the ability of the plant to synthesize methionine, by the synthesis and stability of the methionine-rich protein, and by effects of over-accumulation of the methionine-rich protein on the viability of the transgenic seeds.
An alternative approach would be to increase the production and accumulation of the free amino acid, methionine, via genetic engineering technology. However, little guidance is available on the control of the biosynthesis and metabolism of methionine in plants, particularly in the seeds of plants.
Methionine, along with threonine, lysine and isoleucine, are amino acids derived from aspartate. The first step in the pathway is the phosphorylation of aspartate by the enzyme aspartokinase (AK), and this enzyme has been found to be an important target for regulation of the pathway in many organisms. The aspartate family pathway is also believed to be regulated at the branch-point reactions. For methionine the reduction of aspartyl .beta.-semialdehyde by homoserine dehydrogenase (HDH) may be an important point of control. The first committed step to methionine, the production of cystathionine from O-phosphohomoserine and cysteine by cystathionine .gamma.-synthase (CS), appears to be the primary point of control of flux through the methionine pathway Giovanelli et al. (1984) Plant Physiol. 77:450-455!.
Before the present invention, no plant gene encoding CS was available for use in genetically engineering the methionine biosynthetic pathway. The present invention provides chimeric CS genes for seed-specific over-expression of the plant enzyme. Combinations of these genes with other chimeric genes encoding AK or AK-HDH and methionine-rich seed storage protein provide methods to increase the level of methionine in seeds.